There are numerous radio communication techniques for transmitting digital data over a wireless channel. These techniques are used in mobile telephone systems, pagers, remote data collection or sensor systems, and wireless networks for computers, among others. Most conventional wireless communication techniques modulate the digital data onto either a high-frequency carrier or narrow pulses that are then transmitted via an antenna.
One such technique is ultra wideband (UWB). An UWB system is defined as a system that transmits radio signals over either a bandwidth of at least 20% of its center frequency or a −10 dB bandwidth of at least 500 MHz. One existing unlicensed frequency band for UWB systems is defined by the US Federal Communications Commission as 3.1-10.6 GHz, with a given spectral mask requirement for both indoor and outdoor applications. One widely used signaling technology to utilize such a wide bandwidth is the impulse radio (IR) UWB technology. IR-UWB technology transmits pulses or pulsed waveforms of very short duration. Since these short duration pulses can be transmitted without use of a carrier, IR-UWB systems have the advantages of low complexity, low power requirements, and good time-domain resolution for location and tracking applications. These advantages provide the IR-UWB with great potential to be employed in wireless applications including sensor networks.
Unfortunately, transmitting short duration pulses also introduces a critical problem in the receiver design of an IR-UWB system. In multipath environments, especially non-line-of-sight (NLOS) environments, the received IR-UWB signals consist of a large number of resolvable multipath components (MPCs), the number of which is much higher than in any other wireless communication system. Generally, for the received signals with resolvable MPCs, the Rake receiver can effectively capture the signal energy spread among the MPCs by assigning each MPC a detecting finger. However in order to precisely match the amplitude, phase, and delay of a specific MPC, each detecting finger requires an individual set of channel estimation, multipath acquisition, and tracking operations. The system complexity of the Rake receiver consequently becomes unacceptably high when the number of detecting fingers is large. In order to balance performance and complexity of the IR-UWB system, some Rake receivers only assign a limited number of detecting fingers to the strongest resolvable MPCs. This type of Rake receiver is called a selective Rake receiver.
In order to capture energy from more MPCs while still maintaining low system complexity, a transmitted reference (TR) transceiver has been developed to detect IR-UWB signals in multipath environments. In each frame, the TR transmitter transmits two pulses, a reference pulse and a modulated data pulse, separated by a time duration known by the TR receiver. At the receiver side, the TR receiver correlates the received IR-UWB signals with their delayed version containing the reference pulse to recover the transmitted information bits. Since both the reference pulse and the data pulse suffer the same multipath fading, the reference pulse provides a perfect template to detect the data pulse. Hence the TR receiver does not need explicit channel estimation, multipath acquisition, and tracking operations. The TR receiver can therefore capture energy from more MPCs with a lower system complexity than an IR-UWB system which does not employ delayed reference pulses.
However, the TR technique requires a delay element with ultra-wide bandwidth, up to several gigahertz, in order to provide the delayed version of the IR-UWB signals. This is very difficult to realize in low complexity and low power IR-UWB systems, especially in an integrated circuit.
A different approach to detecting MPCs has recently been developed, in which a slightly frequency-shifted reference (FSR) is used to replace the time-shifted reference in the TR transmitter so as to remove the delay element from the TR receiver. In the transmitter of a FSR system, a reference pulse sequence and one or more data pulse sequences are transmitted simultaneously, but each data pulse sequence is shifted slightly in the frequency domain by multiplying a specific frequency tone. On the receiver side, the reference pulse sequence is shifted by the same set of frequency tones to detect each data pulse sequence. Since the separation of the reference pulse sequence and the data pulse sequence is implemented in the frequency domain rather than the time domain, the receiver does not require a delay element.
However the FSR technique employs analog carriers to shift IR-UWB signals in both the transmitter and receiver sides. This increases the complexity of the FSR transceiver, which weakens the advantages gained from removing the delay element. Furthermore an FSR transceiver has lower performance than a TR transceiver because the FSR transceiver can be affected by frequency inaccuracy caused by oscillator mismatch, phase inaccuracy caused by multipath fading, and amplitude inaccuracy caused by nonlinear amplifiers.
An IR-UWB technique which does not require a delay element and which does not use an analog carrier would provide the performance advantages of a TR transceiver without the complexity inherent in a delay element.